The initiator used plays a decisive role in the curing of polymerizable resins. Upon irradiation, photoinitiators absorb UV or visible light and form the polymerization-initiating species. In the event of radical polymerization these are free radicals. The photoinitiators are divided into two classes based on the chemical mechanism of radical formation.
Norrish type I photoinitiators form free radicals upon irradiation by unimolecular bond cleavage. Upon irradiation, Norrish type II photoinitiators undergo a bimolecular reaction wherein the excited photoinitiator reacts with a second molecule, the coinitiator, and forms the polymerization-initiating radicals by electron and proton transfer or direct hydrogen abstraction. Type I and type II photoinitiators are used for UV light curing; to date almost exclusively type II photoinitiators are used for the visible light range.
UV curing is characterized by a high reaction rate and is frequently used for the coatings of different substrates such as e.g. wood, metal or glass. Thus for example in EP 1 247 843 a UV-curing coating material is described in which type I photoinitiators such as diethoxyphenylacetophenone or acylphosphine oxide are used.
WO 01/51533 describes a UV-curing wood-coating material in which acylphosphine oxides, α-hydroxyalkylphenones or α-dialkoxyacetophenones are likewise used as photoinitiators. Above all, transparent coatings with low layer thickness can be UV-cured due to the low wavelength of the UV light; however, the limit of UV curing is reached with pronounced shading or pigmentation and greater layer thicknesses. Such resins cure only incompletely with UV light. Moreover, with pigmented compositions an absorption range must be found for the photoinitiator in which the pigment absorbs only weakly.
If greater through-curing depths are required, such as for example in the curing of light-curing dental filling materials, visible light is usually used for irradiation. The photoinitiator system most frequently used for this is a combination of an α-diketone with an amine coinitiator as is described in GB 1 408 265.
Dental compositions in which this photoinitiator system is used are disclosed e.g. in U.S. Pat. Nos. 4,457,818 or 4,525,256, wherein camphorquinone is preferably used an α-diketone. Camphorquinone has an absorption maximum at a wavelength of 468 nm. As a result camphorquinone displays a strong yellow colouring with the disadvantage that materials initiated with camphorquinone/amine have a noticeable yellow cast after curing. This is very disadvantageous in particular in the case of bright white shades of the fully polymerized material.
A further disadvantage of type TI photoinitiators is that they lead to the formation of a sticky surface layer upon polymerization. This so-called inhibition layer is attributable to the inhibition of the radical polymerization by oxygen in air.
EP 0 405 786 A2 discloses initiators based on silicon, germanium or tin which are said to be suitable for the mass polymerization of acrylic monomers in an extruder. The initiators are used together with co-catalysts such as tetrabutylammonium fluoride. Silicon-based initiators such as for example 9-trimethylsilylcarbazole are preferred.